In recent years, increasing use has been made of very high purity water in various applications such as the production of semiconductors for solid state electronics, as feed water for nuclear power generators, and in ultratrace analysis. Operators of such systems must be continually alert to the presence of impurities that can impair their operations or product. One of the most useful methods of accomplishing this purpose lies in the monitoring of solution conductivity (or its inverse, resistivity) to indicate the presence of ions in the water. Although conductivity is not specific in identifying the type of impurities present, it is still most useful in many applications since high purity water requires the absence of almost anything conductive, and can actually serve as a valuable backup in assuring proper pH that is notoriously difficult to measure in such systems.
Various instruments are available for measuring conductivity (or its reciprocal value, resistivity) at normal conductivity levels. However, attempts to adapt these prior instruments to measurements of the extremely low conductivity levels encountered in very pure water have not been entirely successful. The main problem lies in the fact that impurities are not the only sources of ions in water. Very small quantities of the water itself break up or disassociate to form hydrogen and hydroxyl ions which are highly conductive. Such minute quantities of water ions are not significant at higher conductivity levels since they contribute little to the overall conductivity. But in high purity water, where conductivity in the range of one micromho per centimeter or less occurs, water ions become the dominant source of conductivity. To further complicate matters, the effects of temperature changes on water ion conductivity happens to be about three times that produced by temperature variations on salt solutions, such as sodium chloride, which is the most common impurity found in pure water systems. Thus, to achieve accuracy, the conductivity measurements must be temperature compensated for the low level salt concentrations at one rate, whereas the water ions require another.
A straightforward technique for achieving temperature compensation for pure water conductivity measurements involves generating a signal value representative of the conductivity of pure water at the measured temperature, and then subtracting this pure water value from the total conductivity being measured by the precision conductivity meter. The remainder leaves a conductivity value representative of that contributed only by the salt ions, which can then be converted to a signal representative of the conductivity for salts at the standard 25.degree. C. temperature. The temperature corrected signal for the salt conductivity can then be added to a fixed value representing pure water conductivity at the standard reference temperature to yield an accurate overall conductivity measurement. However, in subtracting the pure water value from the measured conductivity figure, the effect of any error in the measured conductivity is greatly amplified due to the fact that the pure water conductivity generally represents a substantial portion of the total conductivity. It thus becomes extremely important to avoid measurement errors such as those due to deposits on the electrodes of the conductivity meter probe which is immersed in the pure water sample.
The four electrode conductivity sensor described in this applicant's prior U.S. Pat. No. 4,118,663 issued Oct. 3, 1978, and assigned to the same assignee as the present invention, effectively avoids errors in measuring conductivity due to fouling of the electrodes due to buildup of impurities or oxidation products on the electrode surfaces. In the case of very low conductivity levels encountered in pure water, the need to avoid such errors due to such electrode deposits becomes crucial because such errors can represent a substantial proportion of the total conductivity signal. In the patented four electrode system, the voltage applied to two electrodes from an operational amplifier circuit is automatically adjusted to compensate for voltage drops resulting from electrode deposits. But, as the voltage increases beyond a predetermined level, the system reaches the limit of its ability to compensate for these deposits and the electrodes must then be cleaned to avoid errors.